This invention relates to the control of fluid flow and, more particularly, to a digital fluid flow control system in which the flow rate is independent of variations in downstream pressure.
In a digital fluid flow control system, a plurality of individually actuatable, digital valve elements are interconnected between an upstream manifold and a downstream manifold. The digital valve elements are bistable, i.e., either open or closed, and are actuated by binary electrical signals weighted in value according to a binary code. The cross-sectional orifice areas of the valve elements are also weighted relative to each other according to the binary code. The fluid flow rate from the upstream manifold to the downstream manifold is controlled by opening and closing the valve elements in different combinations, responsive to the binary actuating signals; due to the identical weighting of the binary actuating signals and the orifice areas of the valve elements, the sum of the orifice areas of the open valve elements proportional to the number represented by the binary actuating signals in the binary code.
In industry, closed loop fluid flow control systems are used extensively to automate fluid processes. Typically, a digital computer generates actuating signals for the flow control system responsive to transducers indicating the dynamic conditions in the process. The fluid flow rate through a control valve is a known function of upstream pressure, downstream pressure, orifice area, and fluid characteristics. In a digital fluid flow control system, the orifice area is determined by the states of the digital valve elements and accordingly by the number represented by the actuating signals in the binary code; the principle variables to be measured are upstream and downstream pressure, and in the case of a gas, temperature. The dependence of the flow rate on downstream pressure complicates the calculation of flow rate and the measurements upon which such calculations are based, particularly in the case of a gas, where the flow rate is a complex function of the ratio of upstream pressure to downstream pressure.
It is advantageous from a manufacturing point of view to construct a digital fluid flow control system having a unitary valve body into which the necessary manifolds and access openings for digital valve elements are formed by drillig bores. However, such a construction is not possible with existing radial configurations of digital fluid flow control systems.